The dispersion of particulate matter in various liquid media to yield stable suspensions represents a process of considerable technological importance in such fields as printing inks and paints. It is generally preferred that, consistent with acceptable rheological characteristics, the resulting suspension should have a high level of stability against sedimentation, or settling, of the suspended particulate matter. Alternatively, if settling does occur, the sediment should be redispersible without great difficulty.
Several factors are known to affect the stability of suspensions including size, shape, polarity, charge and density of the particulate matter. Among these the role of size is generally considered to be most important as predicted by Stokes law of sedimentation (or levitation). For practical purposes, if less than about 0.1 micron in size, particles will stay in suspension for many months provided they remain singly dispersed. In fact, a primary objective of most suspension processes is to reduce the size of the particles of the material to be suspended as much as is feasible by mechanical means, such as milling, in the dry state and/or in the suspended state. Milling is often done in the presence of grinding aids or suspension aids to facilitate the generation and/or stabilization of fine particles. Experience has taught that the prevention of agglomeration of primary particles is frequently very difficult.
Three main types of interaction between suspended particles have been defined: (a) electrical (repulsive between like charges); (b) van der Waals; and (c) colloid protection. Broadly speaking, these are influenced by electrical charge, polarity of the medium, and added macromolecules, respectively. The theory of suspension stability is well substantiated for aqueous systems but for non-aqueous systems agreement between theory and practice is less well established.
The conventional approach to achieve improved suspension stability is the addition of "suspension aids". These can be of several different varieties but all seek to render the dispersed particles more compatible with the continuous phase of the dispersion and/or prevent their substantial settling. Conventionally used suspension aids are surfactants and amphiphilic polymers which act by adsorption at the particle/liquid interface. Sometimes a second liquid is added, and usually in appreciable amount, in an attempt to match the polarity of the continuous phase and the particles. Lastly, non-specific thickening of the liquid medium by addition of a suitable polymer is sometimes employed to reduce particle agglomeration and/or settling.
Existing literature teaches that, in contradiction to the present invention, contacting the particulate matter with a second liquid can often have a detrimental effect on suspension stability. The best known case of this is the presence of water in a pigment, such as titanium dioxide, which is dispersed in an organic low polarity liquid. The presence of the water can lead to the formation of rapidly settling agglomerates of the pigment.
Pre-addition of a relatively large amount of a secondary liquid prior to mixing with the primary liquid, or continuous phase, is generally not employed. Belgian Pat. 655,167 discloses a process to improve paint covering power and luster, which involves pretreatment of the pigment with about 40% by weight of water and then with 1% by weight of a surface modifying ingredient, e.g. acetylacetone, followed by heating of the resulting slurry to dryness. Thereafter, this dried, treated powder is dispersed in the dispersed phase.
It has now been found that a process, in which the addition of a suitable secondary liquid to a finely divided powder prior to its blending with the continuous phase is carried out, can materially improve the suspension suitability even though the final concentration of secondary liquid may be so small as to have insignificant influence on the polarity of the continuous phase overall.